Most vehicles in operation today (and many other devices) are powered by internal combustion (IC) engines. Internal combustion engines typically have a plurality of cylinders or other working chambers where combustion occurs. For example, when a driver presses the accelerator pedal, air and fuel are delivered to the working chambers. The fuel is ignited, resulting in combustion that drives pistons within the engine. The pistons in turn are indirectly coupled to the wheels of the vehicle through the drive train such that reciprocation of the pistons causes the wheels to rotate.
When a driver seeks to slow the vehicle down, he/she can depress the brake pedal or simply release the accelerator pedal, which often induces engine braking. Engine braking involves using pumping losses and/or friction within the engine to reduce the speed of the vehicle. For example, if a manual transmission car is kept in gear and allowed to roll down an incline, it would roll substantially more slowly than if the car were in neutral. This is because the wheels of the car are coupled with the pistons in the engine when the car is in gear. As the pistons move back and forth, friction and pumping losses are generated that slow the car down.
There are a wide variety of ways for engine braking to take place. For example, during engine braking some vehicles tend to fire all the working chambers of the engine, but with minimal amounts of air and fuel. In other implementations, the car enters into a temporary mode commonly referred to as deceleration fuel cutoff (DFCO). In this mode, only air, but not fuel, is passed through all the working chambers. This can help improve fuel economy. However, passing too much air through the working chambers of the engine can negatively impact the performance of the catalytic converter.
Another approach, which is described in U.S. Pat. No. 7,930,087 (hereinafter referred to as the '087 patent), involves manipulating the intake or exhaust valves on one or more cylinders to introduce and discharge air from the cylinders. The passage of air generates pumping losses and negative torque for those cylinders. In the other cylinders, air flow is restricted or cut off so that pumping losses are minimized. To increase the amount of negative torque, air can be passed through a greater number of the engine's cylinders.
Although the above approaches work well for various applications, the present invention seeks to provide improved engine braking designs.